As modern humans evolved from their hominid ancestors, their brain development continued with increasing specialization of regions and functions. One hypothesis suggests that the differences between the left and right hemispheres of the human brain can be traced to humans’ simian ancestors swinging through trees. Grasping one limb after another requires the arms to act independently instead of in unison. Perhaps the ancestors of humans began emphasizing the use of one arm over another, encouraging greater neuronal development in the hemisphere that controlled action on that side of the body.
One of the most pronounced differences between brain hemispheres can be observed in dissection of cadavers. The brain region mainly responsible for speech, the planum temporale, is larger in the left hemisphere of two-thirds of human brains. The left-handed nature of language is evident across time and stage of life. Full-term fetuses exhibit larger, speech-related regions in the left hemisphere than in mirror locations on the right hemisphere. The same was true of Neanderthals, according to the telltale marks on the inside of their 50,OOO-year-old skulls made by contact with their gyri and sulci.
The two sexes also experience differences in brain function. Men are more likely to be left-handed, dyslexic, hyperactive, and autistic. Women are more likely to suffer migraines and, on average, have weaker spatial functioning. Women, though, generally outperform men in the fine motor skills of their fingers, and they learn to speak their native language earlier and foreign languages more easily than men. The bottom line, however, is that if you were to look at two brains on a laboratory table-one from a man, and the other from a woman-you probably wouldn’t be able to tell any difference.
In men, the third interstitial nucleus of the hypothalamus typically is twice as big as it is in women’s brains. The hypothalamus is crucial to sexual behavior, as well as regulation of body temperature, eating, and drinking. Furthermore, women’s and men’s brains differ in response to orgasm. PET scans show less activity in a woman’s prefrontal cortex and in a man’s amygdala during sexual climax, while both sexes experience more neuronal firing in the cerebellum.
THE SEXES DIFFER in cognitive ways. A big one involves spatial orientation. Men typically use mental maps, while women prefer landmarks. Men would likely give directions by saying, “Drive north 2.2 miles, turn east, and drive 1.5 miles,” whereas women would more likely say, “Drive toward the mountains until you see the barn, turn right, and go to the pond.” Small wonder that one sex may get frustrated giving directions to the other. Women take the prize for remembering objects’ locations-where are those keys?- while men win at abstract spatial reasoning, such as mentally rotating objects. As a group, men have a wider dispersal of scores on some mental tests.
Much human behavior arises from culture and environment. Some, however, appears to be prewired into the brain. The capacity for language appears to be so strongly encoded that children raised without exposure to any language will make up their own.
Communication is an evolutionary favored social activity that helps humans compete with other animals for resources necessary for life. Similarly, the brain’s ability to process and integrate visual stimuli exists almost immediately after birth. At only a few weeks old, an infant raises its arms to protect itself from the approach of an object. Sight, texture, and size appear to be aspects of object recognition that the brain is prewired to bring together for self-defense.
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